Active capacitance reactance circuit

ABSTRACT

A VARIABLE IMPEDANCE CIRCUIT INCLUDING A CONSTANT CURRENT GENERATOR CONNECTED IN SERIES WITH AN ACTIVE CIRCUIT ELEMENT HAVING FEEDBACK PROVIDED BY A REACTIVE ELEMENT, AND MEANS TO VARY THE GAIN OF THE ACTIVE ELEMENT.

I Jan. 5, 1971 M. EDGE ACTIVE CAPACITANCE REACTANCE CIRCUIT Fi1 ed April15, 1968 CO/VJTA/VT cweleavr GENE/2A me var M United States Patent3,553,609 ACTIVE "CAPACITANCE REACTANCE CIRCUIT Gordon Malcolm Edge,Cambridge, England, assignor to Cambridge Consultants Limited,Cambridge, England, a British company i Filed Apr. 15, 1968, Ser. No.721,414 Claims priority, application Great Britain, Apr. 21, 1967,18,435/ 67 Int. Cl. H03h 11/00 US. Cl. 333-80 4 Claims ABSTRACT OF THEDISCLOSURE A variable impedance circuit including a constant currentgenerator connected in series with an active circuit element havingfeedback provided by a reactive element, and means to vary the gain ofthe active element.

This invention relates to an electrical circuit arrangement by means ofwhich it is possible to vary the effective value of reactance providedby the circuit by varying a voltage applied to the circuit.

According to the present invention there is provided an electricalcircuit including an active circuit element connected in series with aconstant current generator, a reactive element connected to providefeedback between the output and the input of the active element andmeans to vary the gain of the active element whereby the effectiveimpedance presented at an output of the circuit may be varied.

The invention will now be described with reference to the accompanyingdrawings, in which:

FIG. 1 shows a basic circuit which illustrates the principle of thepresent invention,

FIG. 2 shows a more detailed practical circuit arrangement, and

FIG. 3 shows a circuit which is a modification of that shown in FIG. 2.

Referring to FIG. 1 there is shown an active circuit element which isprovided by a pup junction transistor Q having its collector currentsupplied via a constant current generator I A feed-back capacitor C isconnected between the collector and the base of the transistor Q Theoutput terminals T and T of the circuit are thus effectively connectedto a capacitor shunted by a resistor. The nominal value of capacitanceprovided at the output terminals is related both to the capacitance ofthe feedback capacitor C and to the gain of the active circuit element Qand the value of capacitance appearing at the output terminals can byvaried by varying the potential applied to the base of the transistor Qand thus its gain. Two possible ways of varying the potential applied tothe base of transistor Q are disclosed in the practical circuitarrangements of FIGS. 2 and 3 described below.

Referring to FIG. 2 there is shown an arrangement in which thetransistor Q is connected via a transistor Q which is arranged to supplya constant current to the transistor Q The feedback capacitor C isconnected in a feedback path which includes a transistor Q Thetransistor Q is connected in a common collector configuration so as toincrease the effective gain of the active device Q by presenting a highimpedance to the feedback capacitor C. The gain of the active element Qmay be varied by varying a potential applied between the base of thetransistor Q and the terminal T The constant current generator Qmaintains a very high effective shunt resistance which enables thecircuit to be employed in a wide variety of applications with a minimumof effect on any associated circuit.

It can be shown that the ratio of the value of the g 1 CC change inoutput voltage at the terminals T and T to the change in effective valueof capacitance across the terminals T and T is equal to l/k(jwc) where kis a constant related to parameters of the circuit elements used and 1/jwc isthe reactance of the capacitor C. The effective value ofcapacitance across the terminals T and T is therefore k times the valueof the capacitor C. The constant k'includes a factor related to theamplification factor of the transistor Q so that variations of theamplification provided by the transistor Q effectively varies the valueof capacitance seen across the terminals T and T Referring to FIG. 3there is shown a modification of the circuit of FIG. 2 and including acapicitor C and a diode D. A bias voltage applied across the diode D maybe used to vary the forward impedance of the diode D and thus theamplification characteristic of the circuit. It is, of course, necessarythat any feedback current flowing 'via the capacitor C to the diode D issmall compared with the mean current through the diode.

The effective gain of the circuit is controlled by the modification ofthe effective A.C. base currents to the transistors Q and Q, byvariations in the impedance of the diode D The capacitor C isolates thecircuit consisting of the transistors Q Q and Q from the control voltageacross the diode D. The value of the capacitor C must be large comparedwith that of the feedback capacitor C.

There is thus described a circuit by means of which it is possible tosimulate a capacitor, which is variable in value according] tovariations in an external voltage and which is shunted by a fixedresistive component large compared with the capacitor impedance to besimulated.

I claim:

1. An active capacitance reactance circuit comprising, in combination,afirst active circuit element, an output and an input to said firstactive circuit element, a constant current generator including a fixedlybiased further active circuit element connected in series with saidoutput, a capacitive reactive element connected electrically to saidoutput and said input, a second active circuit element connected to saidcapacitive reactive element and to said input in such a manner as topresent a high impedance to the capacitive reactive element, an input tosaid second active circuit element, and means for potential varying theeffective output impedance presented at said output including biasvoltage supply terminals, variable impedance means connected betweensaid supply terminals and a capacitor connected between said input tosaid second active circuit element and one of said bias voltage supplyterminals so that variation in the potential applied between said biasterminals varies the impedance of said variable impedance means andaccordingly said output impedance.

2. An active capacitance reactance circuit as claimed in claim 1,wherein said impedance means is a semiconductor device having at leasttwo terminals, the impedance between which terminals can be varied,connected between the bias voltage supply terminals and in series withsaid capacitor.

3. An active capacitance reactance circuit as claimed in claim 2, inwhich said semiconductor device comprises a diode.

4. An active capacitance reactance circuit as claimed in claim 1,further comprising a diode connected between the bias voltage terminalsand in series with said capacitor, and in which the first and secondactive elements are, respectively, first and second transistors of oneconductivity type and the further active element is a further transistorof opposite conductivity type to said 3 4 one conductivity type, base,emitter and collector elec- 2,948,869 8/1960 Bigelow 33380(T) trodes ofeach of said transistors, the collector electrodes 3,152,309 10/ 1964Bogusz et a1. 33380 of said first and further transistors beingconnected 3,289,119 11/ 1966 Josephs 33380(T) together to constitutesaid output, said capacitive reactive 2,972,120 2/1961 Kireher et a1.331-180X element is connected to the base electrode of the second 63,260,960 7/1966 Bangert 331-177X transistor, and in which the emitterof the second transistor is connected to the base electrode of the firstHERMANN KARL SAALBACH, Pnmary a l e traII i t P. L. GENSLER, AssistantExaminer References Cited UNITED STATES PATENTS 10 U5. C1. X.R.

2,870,421 1/1959 Goodrich 33380(T) 307-295

